The present invention relates to perforating guns for perforating well casing in a hydrocarbon producing well. More particularly, the present invention relates to an improved firing head that can be actuated mechanically or with differential fluid pressure or with absolute fluid pressure to detonate a perforating gun downhole in a well.
In the production of oil and gas from a subsurface geologic formation, well casing is typically installed in a borehole drilled in the formation. To produce hydrocarbon fluids from the formation, the well casing is perforated with a perforating gun containing multiple shaped explosive charges actuated by a firing head. When the firing head is actuated, a primary explosive is detonated and ignites a booster charge connected to a primer cord. The primer cord transmits a detonation wave to the shaped charges, which are activated to create explosive gas jets for penetrating well casing and the surrounding geologic formations.
Existing firing heads are actuated with mechanical, hydraulic, or electrical mechanisms. Certain mechanical firing heads are actuated by dropping or by pumping a weight (termed a "go devil") into the well tubing. The weight moves through the well tubing and impacts a piston to drive a firing pin into an initiator charge. Other mechanical firing heads drop the weight to release a firing pin retainer so that fluid within the well tubing can force the firing pin into the initiator charge. For example, U.S. Pat. No. 4,924,952 to Schneider (1990) disclosed a detonation assembly which was activated with a wireline tool or a weight to mechanically release a fluid biased firing pin into contact with the detonating head.
Differential pressure firing heads react to a differential pressure between the tubing fluid pressure and the annulus pressure in the annulus between the tubing string and the well casing. When the tubing pressure exceeds the casing pressure by a selected amount, the firing gun is activated to detonate the perforating guns. Typically, a lock holds a firing pin in position to prevent premature detonation of the perforating guns. When the tubing pressure exceeds the annulus pressure, the lock releases the firing pin, and the tubing pressure drives the firing pin into contact with the detonator. In U.S. Pat. No. 4,836,109 to Wesson et al. (1989) a differential pressure actuating piston communicated on the high pressure side to a position below the packer and communicated on the low pressure side with an isolated zone in the well. In U.S. Pat. No. 4,509,604 to Upchurch (1985), a differential pressure actuating piston communicated on the high pressure side to a position above the packer and communicated on the low pressure side with the isolated zone in the well.
U.S. Pat. No. 4,911,251 to George et al. (1990), disclosed a firing head actuated by mechanical or hydraulic force generated by the combined operation of three pistons. An actuator piston was impacted by a go devil or actuated with a wireline tool. The mechanical release of the actuator piston released a first firing piston so that the tubing fluid pressure could drive the first firing piston into the initiator charge. Alternatively, the fluid pressure within the tubing could be increased against a second firing piston to drive the second firing piston and the attached first firing piston into contact with the initiator charge. The differential pressure operating against the second firing piston was the tubing pressure minus the lower pressure in a sealed housing recess.
A similar hydraulic actuation concept was disclosed in U.S. Pat. No. 4,969,525 to George et al. (1990), where a differential pressure piston was moved to release a firing pin. The high pressure side of the piston communicated with the isolated well zone, and the low pressure side of the piston communicated with a sealed chamber charged with atmospheric pressure. A mechanical or hydraulic backup firing head was positioned above the principal firing head and was attached to the booster charge with a backup detonating cord. If the backup firing head was hydraulically operated, the actuation pressure could be set at a level different than that of the principal firing head to control the firing sequence of the firing heads.
In U.S. Pat. No. 5,050,672 to Huber et al. (1991) a perforating gun was attached to the tubing string and run into the well without a firing head. A differential pressure firing head was lowered to a position proximate to the perforating gun and could be separately withdrawn from the well if the firing head did not operate.
Electrically actuated firing heads have been used to detonate perforating guns. U.S. Pat. No. 5,115,865 to Carisella et al. (1992) describes different electrical detonation techniques and discloses safety techniques for preventing the untimely detonation of the perforating guns.
In U.S. Pat. Nos. 5,287,924 and 5,355,957 to Burleson et al. (1994), first and second pressure actuated firing heads were positioned downhole in a well, and an actuating fluid pressure was selectively isolated from the second firing head until the first firing head was actuated. This concept provides a technique for the selective perforation of multiple well zones. Actuation fluid pressure for the firing heads of each gun was provided through the bore of the tubing string, and the operating pressure for each firing head was determined by the number of shear pins retaining each firing piston.
In U.S. Pat. No. 5,366,014 to George (1994), a modular perforating gun system permitted the installation, actuation and removal of multiple perforating gun modules conveyed on coiled tubing or other mechanisms.
The use of different style firing heads for perforating guns typically requires the storage and use of mechanical and hydraulic firing heads. The cost resulting from firing head failure encourages secondary backup firing heads in the well, which increases tool string length and well operator cost. Accordingly, a need exists for a combined firing head that can reliably operate by mechanical and hydraulic operation, and that automatically provides backup firing capability.